Rotate does not update dimensions
If you want to check final size after a rotation, you need to apply the rotation: CONTROL+A > APPLY ROTATION

before rotation applied:



after rotation applied:



more info:


  Applying transform values essentially resets the values of object’s
  position, rotation, or scale, but does not actually do anything to the
  object. The object origin point is moved to the global origin and the
  transform values are set to zero. In terms of scale, the scale values
  return to 1.
  
  To apply a transform select the Apply sub-menu from the Object menu or
  use the shortcut Ctrl-A and select the appropriate transform to apply.
  The Operator panel lets you choose the combination of transformations
  to apply.